Fine-scale micro-air bridge fuse

ABSTRACT

Structures of and methods for fabricating fine-scale interconnects and fuses are disclosed. A “mushroom”-type structure with a narrow stalk supporting a wider cap can be used for fine-scale interconnects with widths on the scale of hundreds of nanometers that have low resistivity. Micro-air bridges can be introduced by omitting the stalk in sections of the interconnect, allowing the interconnect to bridge over obstacles. The mushroom-type micro-air bridge structure can also be modified to create fine-scale fuses that have low resistivity overall and sections of significantly higher resistivity where the micro-air bridges exist. The significantly higher resistivity results in preferential fusing at the micro-air bridges. Both mushroom interconnects and mushroom fuses can be fabricated using electron beam lithography.

TECHNICAL FIELD

The present disclosure relates to traces and components in circuits, andmore specifically to fine-scale interconnects and fuses.

BACKGROUND

As solid state electronic display technology moves to smaller scales,formation of interconnects that can address multiple circuit elementsindependently becomes increasingly challenging. This difficulty ismagnified when all interconnects need to be on the same face of thesubstrate and the morphology needs to be maintained as approximatelyplanar. Conventional approaches for creating interconnects, such asphotolithographic techniques have lower bounds on the feature sizes theycan produce due to the wavelength of light that is used to expose theresist. Special techniques, such as e-beam lithography and Damasceneprocesses, can be used to create even smaller feature sizes, but theyare not universally applicable.

High-density interconnects are generally formed by bridging oneconductor over another using a dielectric material to separate the twoconductors. However, even bridging interconnects over one another likethis is limited by pin-hole density, unacceptably increased parasiticcapacitance, and difficulties associated with optically defining finemetal tracks over stepped material. Interconnects with air bridges canbe fabricated using sacrificial layers that support interconnects priorto plating and are later removed, but this requires multiple processingsteps and results in radically non-planar surfaces.

SUMMARY

A “mushroom”-type structure (a narrow stalk supporting a wider cap) canbe used to produce fine-scale low-resistivity interconnects with widthson the scale of hundreds of nanometers. Micro-air bridges can beintroduced by omitting the stalk in sections of the interconnect,allowing the interconnect to bridge over obstacles. The mushroom-typemicro-air bridge structure can also be modified to create fine-scalefuses that have low resistivity overall and sections of significantlyhigher resistivity where the micro-air bridges exist. The significantlyhigher resistivity results in preferential fusing at the micro-airbridges.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary implementations of the present disclosure are described hereinwith reference to the accompanying drawings, in which:

Figure (FIG. 1A is diagram of a first cross-sectional view of a mushroominterconnect, according to one embodiment.

FIG. 1B is a diagram illustrating a second cross-sectional view of amushroom interconnect crossing over an obstacle, according to oneembodiment.

FIG. 2 is a series of cross-sectional diagrams illustrating steps of amethod for fabricating a mushroom interconnect on a substrate, accordingto one embodiment.

FIG. 3 is a series of top-down view diagrams illustrating steps forexposing the substrate to multiple electron beams in a top-down view,according to one embodiment.

FIG. 4 illustrates a top-down view of a mushroom interconnect passingover an obstacle on a substrate, according to a first embodiment.

FIG. 5 illustrates a top-down view of a mushroom interconnect crossingover an obstacle on a substrate, according to a second embodiment.

FIG. 6 illustrates a top-view of a densely-packed array of LED contactsconnected to control contacts via mushroom interconnects, according toone embodiment.

FIG. 7A is a diagram of a first cross-sectional view of a mushroom fuse,according to one embodiment.

FIG. 7B is a diagram of a second cross-sectional view of the mushroomfuse, according to one embodiment.

DETAILED DESCRIPTION

Embodiments relate to fine-scale fuses in electronic circuits. Afine-scale fuse between a first point and a second point on a substratemay include a first section extending from the first point, a secondsection extending from the first section, and a third section extendingfrom the second section to the second point. Each of the first andsecond segments has a stalk segment and a cap segment on top of thestalk segment. A cap segment of the second section is suspended betweenthe first segment and the second segment without a supporting stalksegment so that a layer of air exists between the substrate and the capsegment of the second section. The second section has higher resistivitythan the first and third sections, resulting in preferential melting ofthe second section during periods of overcurrent.

Embodiments also relate to fabricating a fine-scale fuse by usingelectron beams. A first electron beam that can penetrate all resistlayers on a substrate is directed along a path on the substrate butshielded for at least a portion of the path. A second electron beam thatcannot penetrate all the way through all of the resist layers on thesubstrate is directed along the entire path without being shielded.Exposed portions of the resist are removed and then filled with aninterconnect material. The rest of the resist is removed, resulting in afuse structure with a suspended portion that corresponds to where thefirst electron beam was shielded.

Reference will now be made in detail to several embodiments, examples ofwhich are illustrated in the accompanying figures. It is noted thatwherever practicable similar or like reference numbers may be used inthe figures and may indicate similar or like functionality. The figuresdepict embodiments of the disclosed system (or method) for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles described herein.

Example Mushroom Interconnects with Micro-Air Bridges

FIG. 1A is diagram of a first cross-sectional view 100 of a “mushroominterconnect” 110 on a substrate 120, according to one embodiment. Themushroom interconnect 110 has the cross-section of a mushroom gate andis used as an IC interconnect to connect electrical elements (i.e.,electrical contacts). For the purposes of explanation, the mushroominterconnect 110 has two structural components: a stalk 112 with a width114 and a cap 116 with a width 118. Though the stalk 112 and the cap 116are discussed herein as separate parts, they may be continuous andcreated of the same material in the same deposition step, as explainedin conjunction with FIG. 2.

In a typical mushroom gate (which similarly has a stalk 112 and a cap116), the stalk 112 is connected to the active region and its width 114is significantly narrower than the width 118 of the cap 116, which sitson top of the stalk 112 and has a mushroom-like shape (wider on thebottom than the top, potentially with a centrally-located trough alongthe length of the top of the cap 116). This structure results in a shorteffective gate length and low gate resistance. The disclosed mushroominterconnect 110 utilizes these structural components to reduce thefootprint of the interconnect 110 on the substrate 120 and furthermodifies it to include one or more air-bridges, as shown in FIG. 1B. Insome embodiments, the width 114 of the stalk 112 may be 200-300, orspecifically 200 or 250 nanometers. In one embodiment, the width 118 ofthe cap 116 is 500-800 nanometers, or specifically 500 or 750nanometers.

Only the stalk 112 of the mushroom interconnect 110 comes into contactwith the substrate 120. Accordingly, any section of the mushroominterconnect 110 that does not have a stalk 112 instead has a micro-airbridge and does not come into contact with the substrate 120 in thatarea.

FIG. 1B is a diagram of a second cross-sectional view 150 of a mushroominterconnect 110 crossing over an obstacle 130, according to oneembodiment. The second cross-sectional view 150 of the mushroominterconnect 110 is along the length of the interconnect 110 (i.e.,between two or more points being connected by the interconnect 110) andis perpendicular to the first cross-sectional view 100 of the mushroominterconnect 110, which is across the width of the interconnect 110. Theobstacle 130 is an object or area with which the mushroom interconnect110 does not come into contact. The obstacle 130 may be, for example, anelectrical contact to an independently-controlled device.

The mushroom interconnect 110 has at least a first section 142, a secondsection 144, and a third section 146. In the first section 142 and thethird section 146, the mushroom interconnect 110 has both a stalk 112and a cap 116. In the second section 144, the mushroom interconnect 110does not have a stalk 112. Instead, the second section 144 of themushroom interconnect 110 only has a cap 116, which passes over theobstacle 130 without coming into contact with it. The gap between theobstacle 130 and the cap 116 is filled with air, resulting in an “airbridge” in the mushroom interconnect 110 where the obstacle 130 islocated.

In some embodiments, the cap 116 in the second section 144 of themushroom interconnect 110 is raised relative to the cap 116 in othersections 142 and 146 of the mushroom interconnect 110 (as shown in FIG.1B). However, that need not always be the case—in some embodiments, thecap 116 is at the same height in all three section of the mushroominterconnect 110. One of many factors that may affect the height of thecap 116 is the height of the obstacle 130 and the air gap for adequatelyinsulating the obstacle 130 from the mushroom interconnect 110. If thesum of the height of the air gap for insulating the obstacle and theheight of the obstacle 130 is less than/equal to the height of the stalk112, the cap 116 may not be raised to function effectively. In thatcase, the cap 116 may be raised due to specific steps in the fabricationprocess, which is discussed in further detail in conjunction with FIG.2. For example, the resist layer corresponding to the stalk 112 may bedeposited in an even layer that conforms to the topology of thesubstrate 120, including the obstacle 130. In that situation, the airgap is approximately the height of the stalk 112 and the cap 116 israised (relative to the cap 116 in the other sections of theinterconnect 110) by that same height of the stalk 112. In someembodiments, the mushroom interconnect 110 may need to pass over severalobstacles 130, resulting in multiple air bridges in a singleinterconnect 110.

Example Method for Fabricating Mushroom Interconnects

FIG. 2 is a series of cross-sectional diagrams illustrating steps of amethod 200 for fabricating a mushroom interconnect 110 on a substrate120, according to one embodiment. This method includes lithographicprocesses (shown as 210, 220, 230, 240, 250, and 260), and depositionprocesses (as shown as 270 and 280). Tri-layer e-beam resist may be usedin the lithography steps to create high-resolution features on the scaleof hundreds of nanometers. For sections of the substrate 120 where themushroom interconnect 110 will have a stalk 112 (e.g., section 142 and146 of FIG. 1B), the method comprises processes associated with 210,220, 230, 250, and 270 in FIG. 2. For sections of the substrate 120where the mushroom interconnect 110 will not have a stalk 112 (e.g.,section 144 in FIG. 1B), the method comprises processes associated with210, 240, 260, and 280 in FIG. 2.

The substrate 120 is prepared 210 for e-beam lithography by depositionof e-beam resist on the substrate 120. Each of the e-beam resist layers212, 214, and 216 is applied to the substrate 120 using conventionaldeposition methods, such as spin-coating or dip-coating. In someembodiments, the e-beam resist layers 212, 214, and 216 conform to thesurface of the substrate 120, which can result in raised sections of themushroom interconnect 110 where contacts or obstacles exist. Thoughthree e-beam resist layers 212, 214 and 216 are shown in FIG. 2, fewer(e.g., one) or more layers of e-beam resist may be used instead. Anexample tri-layer e-beam resist stack comprises two layers of polymethylmethacrylate (PMMA) resist separated by a layer of copolymer resist. Inanother embodiment, a bilayer e-beam resist stack is used and maycomprise a bottom layer of a PMMA resist and a top layer of copolymerresist, such as a PMMA/methacrylic acid (MA) resist.

The one or more e-beam resist layers 212, 214, and 216 are exposed 220,230, and 240 to beams of electrons that weaken the layers of e-beamresist 212, 214, and 216. For sections of the substrate 120 where themushroom interconnect 110 will have stalk 112, the substrate 120 isexposed 220 to a first electron beam. This first electron beam is highenergy and powerful enough to penetrate through and expose all of thee-beam resist layers 212, 214, and 216. The first electron beam exposesa narrow portion 222 of the e-beam resist layers 212, 214, and 216, theexact dimensions of which may vary based on the desired dimensions ofthe stalk 112 of the mushroom interconnect 110 and limitations of e-beamtechnology. The width of the exposed portion 222 may be substantiallysimilar to the desired stalk 112 width, examples of which are given inconjunction with FIG. 1A.

After being exposed 220 to the first electron beam, the substrate 120 isexposed 230 to a second electron beam that is not as powerful as thefirst electron beam to create the cap 116 of the mushroom interconnect110. The second electron beam is lower energy than the first electronbeam and does not penetrate through and expose the full depth of the oneor more e-beam resist layers 212, 214, and 216. For example, the secondelectron beam may only expose the top two layers 214 and 216 in atri-layer resist stack, or the top layer in a bi-layer resist stack. Thesecond electron beam has a wider, more diffuse footprint than the firstelectron beam, resulting in an exposed portion 232 of the one or moree-beam resist layers 212, 214, and 216 that is both wider and shallowerthan the exposed portion 222. In some embodiments, the second electronbeam is passed over the substrate 120 multiple times in order to fullyexpose portion 232. Specifically, the second electron beam may perform afirst pass over the substrate 120 where it is offset from (i.e., notcentered over) from the exposed portion 222 and a second pass over thesubstrate 120 where it is offset in the opposite direction of the firstpass. This is further discussed in conjunction with FIG. 3.

For sections of the mushroom interconnect 110 that will not have a stalk112, the substrate 120 is not exposed 220 to the first electron beam,and is instead only exposed 240 to the second electron beam. Since thereis no exposed section 222 with which the second electron beam can bealigned for exposure 240, the second electron beam may instead be offsetfrom an axis along the length of the substrate 120 that lines up withthe exposed sections 222 in the sections of the substrate 120 that willhave a stalk 112. Though a “first” and a “second” electron beam arediscussed above, in practice, these may be the same electron beam thatis focused differently such that it has different power and footprintcharacteristics.

The exposed portions 222 and 232 of the e-beam resist are developed andremoved 250 and 260. Removal 250 of exposed portions 222 and 232 resultsin a gap 252 in the e-beam resist corresponding to both a stalk 112 anda cap 116. Removal 260 of the exposed portion 232 results in a gap 262in the e-beam resist that only corresponds to a cap 116.

Finally, an interconnect material is deposited 270 and 280 into gaps 252and, and the remaining portions of the e-beam resist 212, 214, and 216are removed. Possible interconnect materials include conductivematerials such as, but not limited to, platinum, gold, silver, copper,titanium, and layer combinations of these materials.

The remaining portions of e-beam resist layers 212, 214, and 216 areremoved using conventional stripping techniques. The resultingstructures include a section of the mushroom interconnect 110 with botha stalk 112 and a cap 116 that in direct contact with the substrate 120(as shown in 270), and a second section of the mushroom interconnect 110with only a cap 116 that does not come into contact with the substrate120 and is instead separated from the substrate 120 by a layer of air282 (as shown in 280).

FIG. 3 is a series of top-down view diagrams illustrating steps forexposing the substrate 320 to multiple electron beams in a top-downview, according to one embodiment. Views 300, 350, and 360 of FIG. 3illustrate the exposure 220, 230, and 240 from method 200 in the contextof the fabrication of an entire mushroom interconnect 110, instead ofrelative to cross-sections of the mushroom interconnect 110.

For the purposes of explanation, substrate 120 with one or more e-beamresist layers 212, 214, and 216 is collectively referred to as thesubstrate 320 in this figure, even though only the e-beam resist layer216 would be visible from the top-down view. Substrate 320 has an axis310 running along the length of what will be the mushroom interconnect110. Substrate 320 has three sections 342, 344, and 346 that correspondto three sections (e.g., 142, 144, and 146 of FIG. 1B, respectively) ofthe resultant mushroom interconnect 110. The mushroom interconnect 110will have a stalk 112 in the first and third sections 342 and 346,respectively, of the substrate 320. The mushroom interconnect 110 willnot have a stalk 112 in the second section 344 of the substrate 320.Accordingly, steps 220 and 230 of method 200 are performed on the firstand third sections 342 and 346 of the substrate 320 while step 240 ofmethod 200 is performed on the second section 344 of the substrate 320.

Portions of the substrate 320 that correspond to the stalk 112 areexposed 300 along the axis 310 using the first electron beam. The stalk112 exists in the first and third sections 342 and 346, so portion 322 ais exposed in the first section 342 and portion 322 b is exposed in thethird section 346 (both according to exposure 220 of method 200). Forexample, the first electron beam is directed towards the portions of thesubstrate 320 that correspond to the stalk 112.

No portion of section 344 is exposed by the first electron beam becauseno stalk 112 exists there. Instead, section 344 is “shielded” from thefirst electron beam, which in this context means that it is not exposedto the first electron beam. This may be achieved by simply turning offthe first electron beam during that portion of the trace, rather thanphysically shielding section 344 from first electron beam itself. Inembodiments where the mushroom interconnect 110 overlaps with anobstacle 130, section 344 of the substrate corresponds to the areaaround and including the obstacle 130. Examples are further discussed inFIGS. 4, 5, and 6.

Portions of the substrate 320 that correspond to the cap 116 of themushroom interconnect 110 are exposed 350 and 360 along the axis 310using the second electron beam. Because the cap 116 of the mushroominterconnect 110 is continuous and exists in all three sections 342,344, and 346 of the substrate 320, all three sections 342, 344, and 346of the substrate 320 are exposed 350 and 360. In view 350, a portion 332a extending through all three sections 342, 344, and 346 of thesubstrate 320 is exposed by the second electron beam in a first passwhere the second electron beam is offset above the axis 310. In view360, a portion 332 b extending through all three sections 342, 344, and346 of the substrate 320 is exposed by the second election beam in asecond pass where the second electron beam is offset below than the axis310. Exposure 350 and 360 of the first and third sections 342 and 346 ofthe substrate 320 correspond to exposure 230 of method 200. Exposure 350and 360 of the second section 344 of the substrate correspond toexposure 240 of method 200.

In some embodiments, portions 332 a and 332 b overlap minimally, suchthat exposed portions 332 a and 332 b connect but also preferably have aconsistent exposure depth. Exposure depth may be greater in overlapregions due to multiple exposures, resulting in a cap 116 that extendslower in those overlap regions. Exposure depth is primarily aconsideration in section 344, where the deeper exposed portion 322corresponding to the stalk 112 does not exist.

Example Mushroom Interconnects

FIG. 4 illustrates a top-down view 400 of a mushroom interconnect 410passing over an obstacle 430 on a substrate 420, according to a firstembodiment. Mushroom interconnect 410 has a cap 416, and stalk segments412 a and 412 b (collectively referred to as stalk 412). The stalk 412is indicated in dashed lines because it is fully concealed by the cap416. Two contacts 440 a and 440 b and an obstacle 430 are attached tosubstrate 420. Portions of the contacts 440 a and 440 b and the obstacle430 that are concealed by the cap 416 and/or the stalk 412 are indicatedin dotted lines.

The mushroom interconnect 410 has a first section 452, a second section454, and a third section 456; and extends from the first contact 440 aover the obstacle 430 to the second contact 440 b. The cap 416continuously spans and electrically connects all three sections 452,454, and 456 of the mushroom interconnect 110.

The first section 452 of the mushroom interconnect 410 has a stalksegment 412 a that is electrically connected to the first contact 440 aand the cap 116. The third section 456 of the mushroom interconnect 110has a stalk segment 412 b that is electrically connected to the secondcontact 440 b and the cap 116. The second section 454 of the mushroominterconnect 410 does not have a stalk segment and is not electricallyconnected to the obstacle 430. Instead, the second section 454 is amicro-air bridge that separates the cap 116 from the obstacle 430 withair. To avoid all contact with the obstacle 430, the micro-air bridge ofthe second section 454 spans an area larger than the obstacle 430itself. Thus, even the sides of the obstacle 430 are separated from theinner sides of the stalk segments 412 a and 412 b by a layer of air.

FIG. 5 illustrates a top-down view 500 of a mushroom interconnect 510crossing over an obstacle 530 on a substrate 530, according to a secondembodiment. Mushroom interconnect 510 has a cap 516, and stalk segments512 a, 512 b, and 512 c (collectively referred to as stalk 512). Thestalk 512 is indicated in dashed lines because it is fully concealed bythe cap 516. Four contacts 540 a, 540 b, 540 c, and 540 d, and anobstacle 530 are attached to the substrate 520. Portions of the contacts540 a, 540 b, 540 c, and 540 d and the obstacle 530 that are concealedby the cap 516 and/or the stalk 512 are indicated in dotted lines.

The mushroom interconnect 510 has a first vertical section 552, a secondvertical section 554, a third vertical section 556, a first horizontalsection 562, a second horizontal section 564, and a third horizontalsection 566. The mushroom interconnect 510 connects contacts 540 a, 540b, 540 c, and 540 d while passing over the obstacle 530, which has theshape of an upside-down “T.” The cap 516 continuously spans andelectrically connects all sections 552, 554, 556, 562, 564, and 566 ofthe mushroom interconnect 510.

The intersection of the first vertical section 552 and the firsthorizontal section 562 includes a stalk segment 512 a that iselectrically connected to the first contact 540 a and the cap 516. Theintersection of the third vertical section 556 and the first horizontalsection 562 includes a stalk segment 512 b that is electricallyconnected to the third contact 540 c and the cap 516. The thirdhorizontal section 566 includes a stalk segment 512 c that iselectrically connected to the second contact 540 b, the fourth contact540 d, and the cap 516.

The intersection of the second vertical section 554 and the firsthorizontal section 562 does not have a stalk segment and is notelectrically connected to the obstacle 530. Instead, this intersectionis a first micro-air bridge (like section 454 of FIG. 4) that spans anarea larger than the obstacle 530 itself and is separated from theobstacle 530 by air. The second horizontal section 564 similarly doesnot have a stalk segment and is not electrically connected to theobstacle 530. The second horizontal section 564 is a second micro-airbridge that bridges vertically from stalk segments 512 a and 512 b tostalk segment 512 c, forming a structure that is similar to a tunnel, inthat it has stalk segments 512 a, 512 b, and 512 c enclosing it on bothsides for most of its length.

As shown in FIG. 5, the stalk 512 does not need to form a single line(as is the case with mushroom interconnect 410 in FIG. 4). Instead, thestalk 512 can be made up of segments that are offset from the center ofthe interconnect 510 or run in different directions from each other aslong as they are connected by a continuous cap 516. Additionally, thestalk 512 must mechanically support the cap 516 such that it doesn'tcollapse. Thus, the stalk 512 may include additional segments to supportparticularly wide caps 516 and/or to connect additional contacts 540 tothe same interconnect 510.

FIG. 6 illustrates a top-view of a densely-packed array of LED contacts630 a-1 connected to control contacts 640 a-1 via mushroom interconnects610 a-1, according to one embodiment. The LED contacts 630 a-1 areformed on the surface of a substrate 620 and organized into four rowsthat are offset from each other. Due to the size and arrangement of theLED contacts 630 a-1, as well as the width of the mushroom interconnects610 a-1, the LED contacts 630 a-1 cannot be connected to their controlcontacts 640 a-1 using conventional interconnects along simple paths.Instead, the conventional interconnects must be routed around anyobstacles (i.e., other LED contacts 630 a-1). Mushroom interconnects 610a-1, on the other hand, can bridge over the LED contacts 630 a-1 thatform obstacles. Though FIG. 6 is described with LED contacts, these aremerely an example of one application for mushroom interconnects.

Each of the mushroom interconnects 610 a-1 includes a cap, indicated inlight grey, and a stalk, indicated by stripes enclosed by dashed lines(because the stalk is obscured by the cap). Mushroom interconnect 610 aextends from control contact 640 a and terminates at LED contact 630 a.Mushroom interconnect 610 b extends from control contact 640 b, bridgesover LED contact 630 a, and terminates at LED contact 630 b. Mushroominterconnect 610 b does not have a stalk in the section where it bridgesover LED contact 630 a. Mushroom interconnect 610 c extends from controlcontact 640 c and terminates at LED contact 630 c. Mushroom interconnect610 d extends from control contact 640 d and terminates at LED contact630 d. Mushroom interconnect 610 e extends from control contact 640 e,bridges over LED contact 630 d, and terminates at LED contact 630 e.Similar to mushroom interconnect 610 b, mushroom interconnect 610 e doesnot have a stalk in the section where it bridges over LED contact 630 d.Mushroom interconnect 610 f extends from control contact 640 f andterminates at LED contact 630 f.

Mushroom interconnect 610 g extends from control contact 640 g andterminates at LED contact 630 g. Mushroom interconnect 610 h extendsfrom control contact 640 h, bridges over LED contact 630 i, andterminates at LED contact 630 h. Mushroom interconnect 610 h does nothave a stalk in the section where it bridges over LED contact 630 i.Mushroom interconnect 610 i extends from control contact 640 i andterminates at LED contact 630 i. Mushroom interconnect 610 j extendsfrom control contact 640 j and terminates at LED contact 630 j. Mushroominterconnect 610 k extends from control contact 640 k, bridges over LEDcontact 630 l, and terminates at LED contact 630 k. Similar to mushroominterconnect 610 h, mushroom interconnect 610 k does not have a stalk inthe section where it bridges over LED contact 630 l. Mushroominterconnect 610 l extends from control contact 640 l and terminates atLED contact 630 l.

Mushroom Interconnects as Fuses

FIG. 7A is a diagram of a first cross-sectional view 700 of a “mushroomfuse” 710 on a substrate 120, and FIG. 7B is a diagram of a secondcross-sectional view 750 of the mushroom fuse 710, according to oneembodiment. Like a mushroom interconnect 110, the mushroom fuse 710 hasa stalk 712 with width 714 and a cap 716 with width 718. The stalk 712is in direct contact with the substrate 120 and any contacts on thesubstrate 120, while the cap 716 only comes into direct contact with thestalk 712. The mushroom fuse 710 is spilt into three sections 742, 744,and 746. The first section 742 and the third section 746 of the mushroomfuse 710 have include both the stalk 712 and the cap 716, while thesecond section 744 only includes the cap 716 and is separated from thesubstrate 120 by a layer of air, creating a micro-air bridge.

The mushroom fuse 710 is a mushroom interconnect 110 with its dimensionsmodified to create fuse-like properties. Specifically, the stalk 712 iswidened and the cap 716 is shortened relative to the mushroominterconnect 110 in order to increase the resistance of the secondsection 744 of the mushroom fuse 710 relative to the other sections 742and 746 of the mushroom fuse 710 (which include the stalk 712 inaddition to the cap 716). When the current running through the mushroomfuse 710 is too high, the higher resistance of the second section 744causes it to generate heat and melt before other sections 742 and 746 ofthe mushroom fuse 710 do. This causes the second portion 744 of themushroom fuse 710 to break, disconnecting the first portion 742 from thethird portion 746, and breaking any circuit that the mushroom fuse 710is part of The exact current at which the mushroom fuse 710 fails isdependent based on the dimensions of the micro-air bridge and thematerial the mushroom fuse 710 is made of. The mushroom fuse 710 may bemade of the same materials discussed for the mushroom interconnect 110in some embodiments.

The mushroom fuse 710 is fabricated by the same method 200 as themushroom interconnect 110 with the thicknesses of the one or more e-beamresist layers 212, 214, and 216 and the strength of the electron beamappropriately adjusted such that the exposed portions 232 and 222 matchthe desired dimensions. Additionally, the mushroom fuse 710 may be ableto be produced with other lithography processes than e-beam lithography(such as photolithography) in some embodiments.

ADDITIONAL CONSIDERATIONS

The foregoing description of the embodiments of the invention has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the invention to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the invention be limited not bythis detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsof the invention is intended to be illustrative, but not limiting, ofthe scope of the invention, which is set forth in the following claims.

What is claimed is:
 1. A fuse on a substrate comprising: a first sectioncomprising a first stalk segment on the substrate and a first capsegment coupled to a distal end of the first stalk segment, away fromthe substrate, the first stalk segment narrower than the first capsegment along a lateral direction parallel to a surface of thesubstrate; a second section comprising a second cap segment laterallydisplaced from the first cap segment along the lateral direction andconnected to the first cap segment at a first end of the second capsegment; and a third section comprising a second stalk segment on thesubstrate and a third cap segment coupled to a distal end of the secondstalk segment, away from the substrate, one end of the third cap segmentlaterally displaced from the second cap segment along the lateraldirection and connected to a second end of the second cap segment, thesecond stalk segment narrower than the third cap segment, wherein thesecond cap segment is suspended between the first cap segment and thethird cap segment with a layer of air between the substrate and thesecond cap segment during use as a fuse, and wherein a cross sectionarea of the second section is smaller than a cross section area of thefirst section or a cross section area of the third section.
 2. The fuseof claim 1, wherein a length of second section is between 250 nanometersand 30 micrometers.
 3. The fuse of claim 1, wherein the first stalksegment is connected to a first electrical contact and the second stalksegment is connected to a second electrical contact.
 4. The fuse ofclaim 1, wherein a first height of the first and second stalk segmentsis greater than a second height of the first, second, and third capsegments.
 5. A circuit comprising: a first electrical element on asubstrate; a second electrical element on the substrate; and a fusebetween the first and second electrical elements, the fuse comprising: afirst section comprising a first stalk segment on the substrate and afirst cap segment coupled to a distal end of the first stalk segment,away from the substrate, the first stalk segment narrower than the firstcap segment; a second section comprising a second cap segment laterallydisplaced from and connected to the first cap segment at a first end ofthe second cap segment; and a third section comprising a second stalksegment on the substrate and a third cap segment coupled to a distal endof the second stalk segment, away from the substrate, one end of thethird cap segment laterally displaced from the second cap segment alongthe lateral direction and connected to a second end of the second capsegment, the second stalk segment narrower than the third cap segment,wherein the second cap segment is suspended between the first capsegment and the third cap segment with a layer of air between thesubstrate and the second cap segment during use as a fuse, and wherein across section area of the second section is smaller than a cross sectionarea of the first section or a cross section area of the third section.6. The circuit of claim 5, wherein a first height of the first andsecond stalk segments is greater than a second height of the first,second, and third cap segments.
 7. The fuse of claim 1, wherein thesecond cap is normally displaced from the substrate by a first distanceand wherein the first and the third caps are normally displaced from thesubstrate by a second distance less than the first distance.
 8. The fuseof claim 1, further comprising an obstacle element coupled to thesubstrate, the obstacle element passing through the layer of air betweenthe second cap segment and the substrate without physically contactingthe second cap segment.
 9. The fuse of claim 1, wherein each of thefirst cap segment and the third cap segment has a base surface proximalto the substrate and a distal surface that is distal to the substrate,wherein the base surface is wider than the distal surface for each ofthe first cap segment and the third cap segment.
 10. The circuit ofclaim 8, further comprising an obstacle element coupled to the thirdelectrical contact, the obstacle element passing into the layer of airbetween the substrate and the second cap segment without physicallycontacting the second cap segment.
 11. The circuit of claim 8, whereineach of the first cap segment and the third cap segment has a basesurface proximal to the substrate and a distal surface that is distal tothe substrate, wherein the base surface is wider than the distal surfacefor each of the first cap segment and the third cap segment.